Add exclude region marking in Structure step

Users can now draw rectangles on the document image in the Structure
Detection step to mark areas (e.g. header graphics, alphabet strips)
that should be excluded from OCR results during grid building.

- Backend: PUT/DELETE endpoints for exclude regions stored in structure_result
- Backend: _build_grid_core() filters all words inside user-defined exclude regions
- Frontend: Interactive rectangle drawing with visual overlay and delete buttons
- Preserve exclude regions when re-running structure detection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

admin-lehrer/app/(admin)/ai/ocr-pipeline/types.ts

@@ -225,6 +225,14 @@ export interface StructureGraphic {
   confidence: number
 }
 
+export interface ExcludeRegion {
+  x: number
+  y: number
+  w: number
+  h: number
+  label?: string
+}
+
 export interface StructureResult {
   image_width: number
   image_height: number
@@ -232,6 +240,7 @@ export interface StructureResult {
   boxes: StructureBox[]
   zones: StructureZone[]
   graphics: StructureGraphic[]
+  exclude_regions?: ExcludeRegion[]
   color_pixel_counts: Record<string, number>
   has_words: boolean
   word_count: number